1. Field of the invention
The present invention concerns optical telecommunications. It relates to an optical source comprising a light-emitting component, more specifically a laser diode coupled to the end of an optical fiber and intended in particular for monomode submarine links.
2. Description of the prior art
In making up a source of this kind it is important to fix the optical fiber in alignment with the laser diode so as to obtain optimal coupling and minimal variation of coupling with time during use of the optical source.
Document Nos. Wo-A-79/00099 describes a method of coupling a laser diode of this kind to an optical fiber. According to this document, the laser diode is mounted on one of the branches of a U-shaped base and the optical fiber is supported by a fiber-carrier which is itself fixed to the other branch of the U-shaped base. The position of the optical fiber in the fiber-carrier and that of the fiber-carrier on the branch of the U-shaped base are adjusted dynamically, by energizing the laser diode and measuring the power of the signal delivered by the fiber, so as to identify the optimum positional adjustment of the optical fiber relative to the laser diode.
These adjustments are carried out firstly by displacing the optical fiber in the fiber-carrier which is at this stage fixed relative to the laser diode, in a direction Z substantially perpendicular to the surface of the laser to be coupled to the fiber; after this first adjustment the fiber is fixed to the fiber-carrier. Further adjustment is then effected by displacing the fiber-carrier relative to the laser diode, in directions X and Y perpendicular to the previous direction Z. The fiber-carrier is then fixed to the U-shaped base.
The optical fiber is glued to the fiber-carrier and the fiber-carrier is glued to the U-shaped base.
This method of fixing using glue has disadvantages. Specifically, it entails the risk of degassing and pollution of the active optical surfaces, and does not procure sufficient long-term stability and reliability, by virtue of the strong tendency of the organic materials used for such fixings to creep. Also, this fixing method is found to be difficult to employ in practice in the case where the fiber is a monomode fiber the core diameter of which is very small, of the order of a few microns.
U.S. Pat. No. 4,296,998 also describes a method of coupling a laser diode to a fiber. According to this document, the position of the fiber relative to the laser diode is also adjusted dynamically in three dimensions, firstly the direction Z, then the direcitons X and Y; the fixing of the fiber in order to immobilize it in the direction Z and then in the directions X and Y makes use of a gas laser soldering technique. In the embodiment described, the laser diode is mounted on a base featuring a plinth carrying a mass of solder facing the laser diode. An opening through the mass of solder has a diameter which is large relative to the diameter of the fiber, which is inserted into it and can move within it. The appropriate areas of the fiber are previously coated with a layer of metal, to facilitate the adhesion of the solder to these areas. After the position of the fiber is adjusted dynamically, the mass of solder is melted using a gas laser and, on subsequently resolidifying, immobilizes the fiber.
This laser soldering fixing method necessitates appropriate tooling. This operation is also very delicate to implement, because of the proximity of the active surfaces of the laser diode and of the fiber; moreover, this fixing method does not eliminate a slight inaccuracy as to the final positioning of the fiber, due to the expansion and then contraction of the mass of solder during the process of embedding the fiber.
The present invention is directed towards the implementation of an optical source in which the adjustment of the position of the fiber relative to the laser diode is also effected dynamically in three dimensions and in which the relative fixing of the fiber and of the laser diode, while using soldering techniques, requires only very simple tooling and enables the aforementioned disadvantages to be avoided.